Conventionally, a rotation angle detector detects a rotation angle by using a magnetic detector at a periphery of a rotating magnet that generates magnetic flux.
FIGS. 7A and 7B show a conventional rotation angle detector that includes a magnet 1, a first magnetic sensor 4, and a second magnetic sensor 5. The magnet 1 is in a disk shape for generating a magnetic flux in a perpendicular plane to its shaft. The magnetic flux is aligned diametrically from one end of the magnet 1 to the other end. A center of the shaft of the magnet 1 is positioned at a center of a circular periphery of the magnet 1 for rotational movement of the magnet 1. The first sensor 4 and the second sensor 5 are positioned above a surface of the periphery of the magnet 1, and the two sensors are orthogonally positioned relative to the shaft of the magnet 1 for detecting the magnetic flux from the magnet 1. In this manner, the rotation angle detector can detect a wide range of rotation angle.
The magnetic sensors 4, 5 output detection signals relative to the magnetic flux in a shape of sine curve and in a shape of cosine curve, and the detection signals are converted by an inverse-trigonometric calculation unit (a microcomputer) to have a linear characteristic of increasing nature in an interval of 180 degrees. The detection signals converted in this manner are connected for a coverage of 360 degrees. This method of rotation angle detection is disclosed in Japanese patent document JP-A-2003-75108, and diagrams shown in FIGS. 3A, 3B and 3C illustrate the sine/cosine curves and converted linear outputs from the rotation angle detector.
However, the magnetic flux generated by the magnet 1 forms an open magnetic field at the periphery of the magnet 1 as represented by a chain line circle α in FIG. 8. That is, the magnetic flux at the positions of the sensors 4, 5 in a broken line circle β in FIG. 8 is not aligned to a certain direction but in a radially extending from the periphery of the magnet 1. Therefore, a small positional displacement of the first and second sensors 4, 5 caused by mechanical vibration, assembly error or the like results in a enlarged change in the output from the sensors 4, 5. The change in the output from the sensors 4, 5 leads to an amplification of the error in a detected rotation angle.
The defects described above is more practically described with reference to the drawing in FIGS. 9A, 9B and 9C. The coordinates are defined based on X/Y axes in the plane in parallel with the magnet 1, and Z axis aligned with the shaft in the description.
The positional displacement of the sensor 4 in Y axis direction creates displacement in an output wave form A2 in a chain line against the standard output wave form A1 in a solid line as shown in FIG. 9A. The displacement in the output wave form is relatively large even when the displacement of the sensor 4 is only 1 mm. The displacement in the output wave form is also observed when the displacement of the sensor 4 is 1 mm in Z axis direction. The displacement is shown as the wave form A3 in a broken line in FIG. 9A.
The positional displacement of the sensor 5 can be described in the same manner as the sensor 4. That is, the displacement of the sensor 5 in Y axis direction by 1 mm is observed as a large amount of displacement in a wave form B2 in the chained line in FIG. 9B against a standard output wave form B1 in the solid line. The displacement of the sensor 5 in Z axis direction by 1 mm also results in a displacement in the output as shown in a wave form B3 in FIG. 9B.
Error in a detected rotation angle based on the output from the magnetic sensors 4, 5 is shown in FIG. 9C. That is, the displacement of the sensor 4 or 5 by 1 mm in Y axis direction results in a large amount of error in the output of calculation of the detected rotation angle as shown by a curve C1 in a solid line in FIG. 9C. The displacement of the sensor 4 or 5 in Z axis direction by 1 mm also results in error in the output of the detected rotation angle as shown by a curve C2 in a broken line in FIG. 9C.
An improvement over the conventional rotation angle detector is devised as an example shown in FIG. 10A.
The example of the improved rotation angle detector includes a circular magnet 1 in a disk shape with its shaft aligned with a center of rotation of the magnet 1, and a magnetic sensor 4 and a magnetic members 7, 8. The magnet 1 generates magnetic flux from one end toward the other of the disk shape, and the sensor 4 detects the magnetic flux for generating an output relative to the magnetic flux. The magnetic members 7 and 8 are used to form a magnetic flux induction unit 2 for inducing the magnetic flux from one point radially outside of the magnet 1 to the other point. The magnetic sensor 4 is positioned in a gap G between the two magnetic members 7, 8. In this manner, the magnetic sensor 4 is positioned in the magnetic flux from the magnetic member 7 to 8 (or 8 to 7).
The magnetic members 7 and 8 reflect change of the magnetic flux caused by a rotation of the magnet 1 relative to the magnetic members 7, 8, thereby changing the magnetic flux in the gap G. That is, the output from the sensor 4 in the gap G changes correspondingly. The output from the sensor 4 draws a sine curve. Then, an angle calculation unit (a microcomputer) calculates a detected rotation angle based on a liner characteristic of the sine curve in a range of ±60 degrees around a rotation angle of 180 degrees in the output from the sensor 4.
The magnetic flux in the gap G forms a closed magnetic field, thereby preventing the error in the output from the sensor 4 caused by the displacement of either of the position of the magnetic members 7, 8, the position of the sensor 4 in the gap G or the like. United States patent document U.S. Pat. No. 5,164,668 discloses a rotation angle detector having a structure described above.
The rotation angle detector in FIG. 10A cannot detect the rotation angle in a range of 360 degrees as the rotation angle detector in FIG. 7A, because of a limitation of the detection range caused by the nature of the sine curve outputted from the sensor 4. Further, the output from the sensors 4 and 5 in the gap G (sensing surfaces of the sensors 4, 5 are orthogonally arranged) cannot generate the similar output as the rotation angle detector in FIG. 7A. Therefore, the rotation angle detector in FIG. 10A cannot detect the angle in a range of 360 degrees.